Films of PVC are used to package liquids in pouches. Such films may be formed into a tube by sealing them by heat edge-to-edge. Liquids can then be introduced and successive pouches can be formed by pressing opposed surfaces together using "radio frequency" "RF" welding bars and heating the surfaces to above the sealing temperature by radio-frequency. This is known as the form-fill & seal process. The liquids may be concentrated chemicals such as detergents and household cleaning products such as bleach. The RF sealing contact has to be established through liquid contaminated surfaces. For environmental reasons replacement of PVC film is desirable.
The packaging of liquids by a procedure as described involved generally requires a barrier property to ensure good package integrity. Best barrier properties are obtained using polymers having a low content of polar groups which acts against their RF sensitivity. Good barrier films are hence generally--except for PVC--not easily heated by RF frequencies. Poor barrier properties result in an escape of volatile components such as perfume additives but also to a loss of water upon prolonged storage.
The package integrity also requires that the seal be permanent and non-peelable i.e. cannot be peeled apart below the tear strength of the film. This in turns means that RF welding has to be performed with sufficient intensity using appropriate polymers and RF field strengths. Corona treatment can improve printability but may reduce seal strength, film shelf-life and worsen blocking.
The films used should be processable at high speeds. This in turn means that the film should be non-blocking and does not stick to any equipment used for controlling it on the packaging line. Materials having a high polar group content have low melting points and can be sealed but have poor barrier characteristics and tend to stick and block.
The high processing speeds also necessitate that the RF-welding should proceed quickly without sparking. In industry lower RF frequencies such as 27 MHz are commonly used. The content of polar groups should preferably be sufficiently high to give fast, proper sealing at lower frequencies.
Films may also contain slip agents and fillers to modify their characteristics to reduce blocking for example but RF weldability and printability can be adversely affected. Excessive levels of slip agent may cause the agent to migrate to the surface during storage and affect shelf-life in a commercially unacceptable manner. Migrated slip agent fouls the processing line and may prevent the formation of a permanent seal in RF welding.
Although a number of proposals have been made discussed below for replacing PVC film on RF-sealing packaging lines, none has achieved commercial acceptance up till now as far as inventors are aware. It has not been possible to devise a film which resolves the above contradictions and combines non-blocking, high RF sealability and seal strength and good barrier properties.
GB 2177974 (DRG) uses an inner layer of preferably 18 to 28 wt % vinyl acetate (VA) EVA polymer and outer layers of less RF absorbent material. The low VA content in the outer layers reducing sticking and blocking. However the sealing temperature range is reached using heat transfer between inner RF heatable layer and outer seal layers which will tend to reduce speed of operation. The central high VA layer has poor barrier characteristics.
U.S. Pat. No. 4,678,713/EP 222900 (Dow) and other related documents use high VA outer layers for RF welding, but high enough VA contents to provide fast RF welding cannot be processed at high line speeds because of high friction and blocking.
EP 13745 (BRDR Schur) discloses in column 4 lines 49-54 the use of film covered on one or both sides with EVA which can be RF welded while the base layer may be polyethylene. While widely varying VA contents are indicated EVA's with 16 mol % VA are recommended especially. Such EVA's have low crystallinity and lack sufficient strength to provide a permanent seal. The result is a peelable film seal not a permanent seal necessary to provide high package integrity under mechanical stress. JP 55059921 (Mitsui) discloses EVA as an RF sealable material. JP 53091989 (Asahi-Dow) incorporates PVC and EVA. 83 JP 031755 discloses a complex RF sealable structure. Dow U.S. Pat. No. 4,601,948; EP 223838 (U.S. Pat. No. 4,787,194); EP 223838 (WO-8607034) also disclose EVA materials. U.S. Pat. No. 4,539,793 (Johnson+Sons Inc) discloses an RF sealable packaging construction.
Other EVA film disclosures include FR-2031801; EP-142315 (Grace) and EP 342822 and EP-345927 where a matted surface is provided by embossing for use in the baling of rubber.
It is the object of the invention to provide a non-PVC film which has a combination of properties to provide a commercially acceptable PVC film replacement.
It is in particular an object of the invention to provide a non-PVC film which has a good barrier property, which can give a permanent seal by RF welding and/or which is non-blocking, processable and storable.